Pulse generating compensation circuits in magnetic thin film devices



Oct. 8, 1968 J. B. JAMES PULSE GENERATING COMPENSATION CIRCUITS IN MAGNETIC THIN FILM DEVICES Filed July 14 1964 CURRENT SOURCE A-r-rozusv S United States Patent 3,405,401 PULSE GENERATING COMPENSATION CIRCUITS IN MAGNETIC THIN FILM DEVICES John Bernard James, London, England, assignor to International Computers and Tabulators Limited Filed July 14, 1964, Ser. No. 382,574 Claims priority, application Great Britain, July 19, 1963, 28,661/ 63 7 Claims. (Cl. 340-174) ABSTRACT OF THE DISCLOSURE In thin magnetic film devices, wherein the thin films are supported on an electrically conducting substrate, and are controlled by the external magnetic fields generated by the energization of drive conductors magnetically linked with the film, it has previously been understood that each energization produces eddy currents in the substrate which reinforce the applied magnetic field acting on the film. In pulse-operated devices of this kind the eddy current componentof the total magnetic field has been found to decrease with time. There is disclosed an arrangement in which drive currents for the conductors are supplied by a pulse generating circuit which includes, in series with a drive conductor, a circuit comprising a parallel resistor/ inductor combination having a time constant approximately equal to the time constant of the eddy currents. Under these conditions the amplitude of drive current pulses in a serially-applied train is varied to compensate for the changes in the effective magnetic fields which would otherwise be produced due to the eddy current variations.

This invention relates to pulse generating circuits for the operation of magnetic film devices.

The construction of magnetic thin film devices by the deposition of magnetic alloy films on supporting substrates is well known. Usually such devices are provided with at least one driving conductor which is energised to apply a magnetic field to the film.

It has been proposed in British patent specification No. 942,549 to use an electrically-conductive substrate or backing layer for such a magnetic thin film device in order to reduce the current necessary to drive the film and to reduce the inductance of the driving conductor. The reduction in drive current is effected by the production of eddy currents in the conductive substrate or backing layer when the driving conductor is energised. These eddy currents, themselves, produce a magnetic field which links with the film and which adds to the magnetic field produced by the driving conductor, thereby, in effect, doubling the magnitude of the applied magnetic field.

In some applications of such thin film devices, it is necessary to apply a train of pulses to the driving conductor, the separation of the pulses being comparable with their length. It has been found that, under such conditions, the field component produced by the eddy currents decreases with time, thereby reducing the total effective magnetic field. It is, however, important that the total magnetic field should be substantially the same for all pulses in the train and should depend as little as possible upon the length of the driving pulse train, the repetition rate of the pulses, and their mark/ space ratio.

The object of the present invention is to minimize the changes in the driving magnetic field applied to a magnetic film due to changes in the eddy current component.

According to the present invention, a pulse generating circuit for applying current pulses to a driving conductor of a magnetic film device employing a magnetic film mounted on an electrically conductive layer, includes a pulse source arranged to produce current pulses of magnitude substantially independent of disturbances in the driving conductor, and a compensating circuit comprising an inductor and a resistor connected in parallel, said compensating circuit being connected in series with the pulse source, and the values of said inductor and resistor being such that the time constant of the series circuit including said pulse source and said compensating circuit is approximately equal to the time constant of the eddy currents in the electrically conductive layer of the magnetic film device, whereby the amplitude of pulses of a pulse train applied to said driving conductor varies with time such that changes in amplitude of a magnetic field produced by said eddy currents as successive pulses of said train are applied, are substantially compensated.

The series circuit may also include the driving con ductor.

The pulse source may include a transistor and a current determining resistor connected in series, which resistor may be, at least in part, a terminating resistor for the driving conductor.

The transistor, the compensating circuit, the current determining resistor and the driving conductor may all be connected in series across a voltage supply, and a further transistor may be included as an alternative to the previously-mentioned transistor, the transistors being arranged to cause current to flow in opposite directions, respectively, through the driving conductor.

The base and emitter electrodes of the transistor, or each transistor, may be connected together by way of a secondary winding of a pulse transformer.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which shows a magnetic thin film element having a driving conductor connected to a pulse generating circuit in accordance with the invention.

Referring to the drawing, an anisotropic magnetic thin film element 1 is deposited on a substrate 2 of high purity aluminium. The element 1 has hard and easy axes of magnetisation indicated by arrows 3 and 4, respectively. Driving conductors 5 and 6 are magnetically coupled to the element 1 and are aligned substantially parallel to the hard and easy axes, respectively.

A current source 7 is connected to one end of the conductor 6 and a terminating resistor 8 connects the other end of the conductor 6 to ground.

A pulse supply circuit 9 is connected to one end of the conductor 5, and a terminating resistor 10 connects the other end of the conductor 5 to ground. The terminating resistor 10 is of sufiiciently high value to make the current flowing through the conductor 5 substantially independent of any spurious voltages or other disturbances in the conductor 5. The circuit 9 includes an n-p-n transistor 11 having a collector electrode connected to a terminal 12 which is at a positive potential relative to ground. The emitter electrode of the transistor 11 is connected to a common point 13, and the base and emitter electrodes are connected together through a secondary winding 14 of a pulse transformer 15 which also has a primary winding 16.

A second n-p-n transistor 17 has its collector electrode connected to the common point 13 and its base electrode connected to its emitter electrode through a secondary winding 18 of a further pulse transformer 19 which also has a primary winding 20. The emitter electrode of the transistor 17 is also connected to a terminal 21 which is at a negative potential with respect to ground.

Connected between the common point 13 and the conductor 5 is a compensating circuit 22 comprising an inductor 23 and a resistor 24 connected in parallel.

In operation of the apparatus, in order to store a digit of one binary significance in the element 1, a hard axis magnetic field is applied to the element 1 by energisation of the conductor 6 with a current pulse from the source 7. At the same time an operating signal is applied to the primary winding 16 to cause the transistor 11 to bottom, thereby passing a current through the conductor 5, and causing an easy axis magnetic field to be applied to the element 1. On termination of the current pulse applied to the conductor 6, the magnetization vector of the element 1 adopts the same direction as the easy axis field, and, on termination of this field, remains in that direction.

In order to store a digit of the other binary significance, the above operation is repeated but in this case an operating signal is applied to the transformer primary winding 20, causing the transistor 17 to bottom instead of the transistor 11. A current then flows in the opposite direction through the conductor 5, and an easy axis field in the opposite direction is applied to the element 1. On termination of the driving currents, the magnetization vector then adopts the opposite direction along the easy axis to that mentioned above.

During the energization of the conductor 4, eddy currents are produced in the substrate 2 and these eddy currents themselves produce a magnetic field which increases the effective easy axis field. In the absence of the compensating circuit 22 it has been found that, on application of a train of operating pulses to either of the transformers 15 or 19, the total effective easy axis field decreases with successive pulses, due to a decrease in the eddy current field component. The inclusion of the circuit 22, however, compensates for this decrease and maintains the total field substantially constant in the following manner. The value of the resistor 24 is chosen in dependence upon the mark/space ratio of the applied pulses and the total resistance in the whole series circuit, including any resistance which may be included to overcome changes in transistor parameters. The value of the inductor 23 is the chosen such that the time constant of the parallel combination of the inductor 23 and the resistor 24 is approximately equal to the eddy current time constant, this being calculated in the usual way from the rate of decay of the eddy currents. A typical example of eddy current time constant for one design of thin film device using an aluminium substrate is approximately 1-2 microseconds.

If a train of operating signals is now applied to the compensated pulse supply circuit 9, the current through the conductor will gradually increase due to the circuit 22, and this increase in current, and hence in applied field, will tend to overcome the decrease in the eddy current field component, thereby maintaining the total field substantially constant.

The compensated circuit will clearly need a somewhat higher supply voltage than will a similar circuit without the compensating circuit 22, due to the resistance of the circuit 22.

Although the invention has been described above as applied to a single magnetic thin film element, clearly pulse generating circuits according to the invention may also be used to drive the digit lines of a magnetic film matrix, such as that described in British patent specification No. 942,561. Furthermore, other modes of operation of the film elements may be used. For example, the driving conductors 5 and 6 may be aligned at a small angle to the axes instead of parallel to them. In this case, energisation of the conductor 6 is alone suflicient to set the element in one stable state, and energisation of this conductor together with energisation of the conductor 5 in the correct direction sets the element in the other stable state. Only a uni-polar drive is then required for the conductor 5 and one or the other of the transistors 11 or 17 is then unnecessary.

Although the substrate is said to be of aluminium, clearly other electrically conductive materials may be used without affecting the present invention.

Each pulse source in the above embodiment includes a transistor 11 or 17 and a terminating resistor 10. However, other forms of current pulse source could be used, provided that the current pulses produced have an amplitude which is substantially independent of any spurious voltages or other disturbances which may occur in the conductor 5.

Clearly, also, the circuit for operating the transistors 11 and 17 is of no importance to the invention and any other suitable operating circuit may be used. For example, the two windings 14 and 18 may be on a common pulse transformer.

The compensating circuit 22 may be used at other points in the circuit instead of being connected directly to the drive conductor as in the above embodiment. The amplitude of the drive current may be determined at an earlier pcint in the driving circuits by connecting the compensating circuit 22 at that point.

What is claimed is:

l. A magnetic film device, including an electrically conductive substrate; a magnetic film supported on said substrate; at least one driving conductor lying across and magnetically linked with said magnetic film; and a driving circuit for applying a train of current pulses to the driv ing conductor, the application of said current pulses producing eddy currents having a time constant in said substrate, said eddy currents producing a magnetic field linking with the film; said driving circuit including a pulse source arranged to produce said train of current pulses of magnitude substantially independent of disturbances in said driving conductor; and a compensating circuit having a time constant; said compensating circuit being connected in series with said pulse source comprising an inductor; and a resistor connected in parallel with said inductor, the values of said inductor and resistor being such that the time constant of the driving circuit is approximately equal to the time constant of said eddy currents; whereby the amplitude of pulses in said pulse train varies with time such that changes in amplitude of said magnetic field produced by the eddy currents, as successive pulses of said train are applied, are substantially compensated.

2. A magnetic film device as claimed in claim 1, in which said pulse source includes a first transistor and a current-determining resistor connected in series.

3. A magnetic film device as claimed in claim 2, in which a terminating resistor for said driving conductor forms at least a part of said current determining resistor.

4. A magnetic film device as claimed in claim 3 in which said pulse source includes second switching transistor, and means for connecting said second transistor in series with said current-determining resistor, said first and second transistors respectively being arranged in circuit to cause current to flow in opposite directions through said driving conductor.

5. A magnetic film device as claimed in claim 4 further including at least one pulse transformer having a secondary Winding in which the base and emitter electrodes of at least one of the first and second transistors respectively are connected together through the secondary winding.

6. A pulse generating circuit for applying current pulses to a driving conductor of a magnetic film device employing a magnetic element mounted on an electrically conductive layer, in which layer eddy currents are generated when said conductor is energized, said pulse generating circuit including a pulse source operable to produce current pulses of a magnitude which is independent of disturbances in said conductor; a compensating circuit; means to connect said pulse source and said compensating circuit together in a series circuit, said compensating circuit including a parallel combination of an inductor and a resistor having such values that the time constant of said series circuit is approximately equal to the time constant of said eddy currents; means to apply a train of operating signals to said pulse source to cause said series circuit to produce output pulses which, due to the time constant of said series circuit, having successively increasing amplitudes; and means to couple said series circuit to said conductor.

7. A pulse generating circuit as claimed in claim 6 in "which said pulse source comprises a switching transistor and a current determining resistor which minimizes the elfect of any disturbances in said conductor on the amplitude of the pulses produced by said source; and in which said operating signal train applying means applies said operating signals to said transistor to cause said transister to conduct to produce a corresponding train of output pulses.

References Cited UNITED STATES PATENTS BERNARD KONICK, Primary Examiner.

J. F. BREIMAYER, Assistant Examiner. 

